About Me

Australian philosopher, literary critic, legal scholar, and professional writer. Based in Newcastle, NSW. Author of FREEDOM OF RELIGION AND THE SECULAR STATE (2012), HUMANITY ENHANCED (2014), and THE MYSTERY OF MORAL AUTHORITY (2016).

Monday, August 31, 2009

The Grand Opening up of the Solar System

Baby boomers like me who came of age in the 1970s and date their first reading of science books to the 1960s learned that Mercury is in a tidally-locked orbit around the Sun, so that one side is always in ferocious sunlight while the other is exposed to the near-absolute zero of space. By the mid-60s, that was known to be untrue, but it took years before the truth percolated into popular culture. We learned that Jupiter has twelve moons (over sixty have now been counted) and Saturn nine (we now count 61, in addition to the matter making up the sixth planet's rings).

The nine planets were Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto - that had been the situation since Pluto's discovery in 1930, when our parents were young. But in 1978, the astronomical dot of Pluto was resolved into the binary system of Pluto and Charon, two relatively small bodies in orbit around a common center. Pluto itself has turned out to be far smaller than we baby boomers were taught. By the early 1970s, initial estimates had been scaled down, and it was thought to be comparable in mass to Mars. Certainly, we thought, it was larger than Mercury.

In fact, it is far smaller than Earth's moon and only about a twentieth of the mass of Mercury - the next smallest of the "nine planets" we were taught about.

Until very recently, astronomy needed no formal definition of a planet, but this has changed as our knowledge of the Solar System has increased. During the 1990s we discovered a toroidal region of space known as the Kuiper Belt, which contains not only Pluto but many other objects of similar composition and with similarly unusual orbits when compared to those of the eight larger planets. With a better understanding of the Solar System, astronomers came to understand Pluto as the largest of these Kuiper Belt objects, all of which are very different from any of the other eight planets, and much smaller. Astronomers began to find large objects even beyond the Kuiper Belt, all contributing to what I call the Grand Opening Up of the Solar System.

Against this background, astronomers sought a more precise method of classifying the various large objects that had been identified in orbit around the Sun. Any definition of a planet that included Pluto but definitively excluded the smaller, but similar, Charon, and disqualified such bodies as Quaoar, Sedna, and Eris, would seem arbitrary. Eris, discovered in early 2005, lies beyond the Kuiper Belt in a region of space referred to as the Scattered Disc. It is significantly larger than Pluto, and would surely qualify as a planet if Pluto continued to qualify.

There is an exciting story to be told about the Grand Opening Up of the Solar System, how it led to efforts in 2006 to develop definitions of such categories as "planet", and how it still goes on. A well-informed science journalist with good publishing connections could get a wonderful book out of this story, in the process telling the public much about contemporary astronomy and why the study of our own Solar System is currently in such a wonderful ferment. I'd like to read that book. It could explain how the category "dwarf planet" was invented for bodies such as Pluto and Eris, why some large bodies have not yet been classified until we know more about them (Charon is one of these, as is Vesta, the second largest body in the Asteroid Belt between Mars and Jupiter), what plans are afoot to investigate these bodies, and much else.

Unfortunately, I can't imagine Chris Mooney and Sheril Kirshenbaum writing that book. Where I see excited astronomers responding rationally and reasonably to the Grand Opening Up of the Solar System - refining the categories and definitions that they use in their work - they see a bunch of mean scientists taking an opportunity to give the public a poke in the eye by taking away its beloved ninth planet. This is a pity. They could have done some positive communication here, in the opening chapter of Unscientific America. Instead, they produced a dull and inaccurate narrative that is meant to support their theory that out-of-touch (or even mean-natured and anti-populist) scientists are largely to blame for America's alarming degree of scientific illiteracy.

What a waste of a great opportunity to practice what they preach, and improve the public's understanding of what is really going on in science.

45 comments:

Yes...You see, this very blog post is orders of magnitude more interesting than what M & K write about Pluto - it's interesting, and exhilarating, and leaves one wanting to find out more. (About science - just what M & K profess to want us to want to find out more about!) It's interesting to learn about how knowledge grows and changes, it's interesting to learn about what experts thought 40 years ago and what new knowledge has come in since, and how it has come in, and how the understanding has changed. It's fucking fascinating. But to the hack mind - it's not.

I think there were 21 moons of Saturn when I first started reading science books. What with all the aches and pains I've endured in the past couple months and the unaccountably long grey hairs I keep finding on my head, I appreciate how young this makes me feel.

Tyson's book represents only one side of this ongoing controversy, a point you fail to mention. And Mooney and Kirschenbaum are right to criticize the IAU decision, which was political, not scientific.

Pluto is still a planet. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto.

One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies.

Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.

Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity--a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. This fact makes Pluto, Ceres, Haumea, Makemake, Eris, and possibly Vesta more similar to the planets than to shapeless asteroids and KBOs. What we needed was not to downgrade Pluto but to add dwarf planets as a third subcategory of planets.

These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned. I am a writer and amateur astronomer and proud to be one of these people. You can read more about why Pluto is a planet and worldwide efforts to overturn the demotion on my Pluto Blog at http://laurele.livejournal.com . And I am in the process of writing a book about why the 2006 definition was wrong and supporting the concept that it is perfectly fine to have a solar system with 50+ planets.

Even Tyson at this point is distancing himself from the IAU decision, which he describes as "flawed," admitting that maybe it is too early in the field of planetary science for anyone to be defining the term planet.

Meanwhile, here are some good books representing the pro-Pluto side: "Is Pluto A Planet" by Dr. David Weintraub, and "The Case for Pluto," by Alan Boyle, which will be released in October.

maybe it is too early in the field of planetary science for anyone to be defining the term planet.

I can't see how this works. Science needs operational definitions. So, we have to define a planet, even if the definition has blurred boundaries and isn't intuitively or politically pleasing. I'm not interested so much in what the dictionary says, just what the relevant scientists say. They may wholeheartedly agree with your post, but it seems inescapable that they must still have some definition to work with.....

Yes, you do all need to read The Pluto Files - I decided to read it in the aftermath of the M&K affair and it is, indeed, excellent. The key point that it makes, indeed, is not whether Pluto is-or-is-not a planet, but rather, the pedagogical poverty of teaching the solar system as My-Very-Excellent-Mother-Just-Served-Us-Nine-Pizzas compared to learning about the variety of the solar system objects and to compare and constrast the different families of objects.

And, yes, Laurel is basically Beetlejuice, but with Pluto and no particular need for repetition (maybe given this discovery, we need to come up with a new, stronger definition of Beetlejuices?).

As I said in the last thread, the pertinent issue here is not what the definition of a planet should be, but whether the treatment in Unscientific America is appropriate and accurate. Maybe you want Pluto to be a planet, Ceres to be a mesoplanet and Earth to be a giant wet asteroid. Whatever. That's not the point here.

No definition of "planet" will satisfy everyone, for the simple reason that we're trying to stretch a limited set of words over a heterogeneous class of objects which can each be measured in many ways. It's like a blanket which is too small for the bed: no matter which way you pull it, some part of you is going to be cold. Take the issue of "hydrostatic equilibrium": it doesn't depend only on mass, but also on geological composition.

[I]magine two planets which are the same size, but one is solid rock and the other is a less solid rock. It's easy to imagine the first being able to retain a lumpy shape due to its structural strength, while the other one doesn't, even though it's the same overall size and mass! [...] Ceres is the largest asteroid, and big enough that its own gravity can crush it into a sphere. No other asteroid is big enough or massive enough to do that, but several of the larger asteroids are massive enough that their shapes are modified by gravity, and are "nearly round". Why aren't they on the list? Maybe they're not at hydrostatic equilibrium, and geological forces dominate over gravity (in other words, the gravity isn't enough to totally morph the thing into a ball). But this seems like another somewhat arbitrary line in the sand. How far away from a perfect sphere does an object have to be before it's tossed out of the planet club?

Sometimes, history leaves us with terms which don't particularly make sense: a "planetary nebula" has nothing to do with planets, most "minor planets" aren't planets by anybody's definition, and "asteroids" aren't actually "starlike".

Sorry Ophelia, Joel, and El Rey, but many people do care about Pluto, and I, as an amateur astronomer, am one of them. Personal attacks don't do anything to support your arguments. If you're so keen on making personal attacks, why not target Mike Brown, who calls himself "Plutokiller" and is obsessed with the idea that he killed Pluto and reshaped the solar system, a person who in spite of all evidence to the contrary, denies there is an ongoing debate.

Your double standard is outrageous. It's okay for those who agree with you to write whole books about Pluto, but if someone who disagrees with you does the same thing, it's an "obsession?" I'm not going to stop writing about Pluto, no matter how rude some people are. When the time comes, you should give my book the same chance as you do Tyson's.

Brian, the IAU definition is not an "operational definition." It does not at all address exoplanets, many of which have inclined orbits, with at least one star system that has two giant planets orbiting in a 3:2 resonance just like Neptune and Pluto. It makes no sense to have one definition of planet for this solar system and a different definition (or no definition) for other solar systems. The continued discovery of so many unusual exoplanets is why Tyson said it may be too early to be making such definitions. The point is, a bad definition is worse than no definition.

Blake, the IAU definition satisfies a very limited number of people. The fact that so many professional astronomers oppose it speaks to that clearly. The writers of "Unscientific America" made the valid point that the process by which this definition was adopted as well as the definition itself are so flawed and problematic that they represent the worst face of scientists and turn people off to science.

The size at which an object attains hydrostatic equilibrium does not vary greatly even in the case of different geological compositions. Ceres and Pluto are clearly planets, as they are without doubt in hydrostatic equililbrium. No matter what definition is used, there will always be some objects at the boundaries, not quite in one category or the other. Maybe we need another subcategory for these objects, such as "sub-dwarf planets." After all, we have brown dwarf stars and sub-brown dwarf stars, so there is precedent for this.

Stephen, when I said that asteroids are "shapeless," I meant that they are lumpy with shapes determined solely by chemical bonds. That differentiates them from objects large enough to be rounded by their own gravity. The latter have geological processes just like Earth does and many also have weather.

While I've met Tyson and respect him, there are several flaws in his book. First, he fails to indicate that the IAU definition states that dwarf planets are not planets at all, which makes no sense and is inconsistent with the use of the term dwarf in astronomy. He is also wrong in calling Pluto a "large comet." Pluto is different from most Kuiper Belt Objects in one crucial way, specifically that it is in hydrostatic equilibrium.

The writers of "Unscientific America" made the valid point that the process by which this definition was adopted as well as the definition itself are so flawed and problematic that they represent the worst face of scientists and turn people off to science.

No, they didn't make that point. They failed to describe the history of the decision-making process, they elided the ways scientists used the status of Pluto to provoke beneficial interest in astronomy, and they failed to demonstrate that people were actually turned off from science.

Alan Stern of the IAU and NASA's New Horizons mission to Pluto recently debated Mr. Tyson. Not all planetary scientics are in the same camp as Mr. Mike "plutokiller" Brown and Mr. Tyson. If Pluto with three moons and an orbit that is sometimes closer to the Sun than Neptune can be demoted, can't Earth and Neptune be demoted, too? When will this stop? 424 of 10,000 IAU members voted on this absurdly tilted vote on the last day of the 2006 IAU General Assembly and in the 2009 General Assembly it was admitted de facto that the vote was a sham.

Laurel, no one is "personally attacking" you. They're criticizing your pedantic, overblown, too-long disquisitions on this subject. You really do appear to people who don't know you as disproportionately obsessed with this.

The bear analogy fails miserably. Lots of animals are called "bears" that are not members of the family Ursidae - koala "bears", "bear" cats and "bear" dogs and even water "bears" and woolly "bears". Just because something is called a "planet" doesn't mean it is a planet.

"Just because something is called a "planet" doesn't mean it is a planet."

Huh? I'm not sure what this sentence is supposed to mean. The term "dwarf planet" is a noun modified by an adjective. Certain terms, like bear cat, are misnomers and in many cases have been replaced. "Bear cats" are now referred to as Binturongs. When creating new terms, if the goal is to clarify distinctions between categories, the worst thing to do is to make up a term that blurs such distinctions and does not follow grammatical conventions and that in fact goes against other uses of that same term (dwarf) in the field (astronomy, in this case).

Josh S., I may be wordy, but it really doesn't matter how I "appear" to certain people. What matters is that those upholding a particular viewpoint, namely that Pluto is not a planet, want to stifle debate and shut up anyone who vigorously and openly dissents with their view. This is exactly what the IAU did at its General Assembly this year.

Interestingly, I've met Tyson; he autographed a copy of his book for me, and we were able to laugh and joke about our differences. I don't know why others cannot do this.

I'm pursuing a degree in astronomy and have as much right to discuss this online as anyone else, including proponents of the IAU view. My professional goal is to see a complete revision of the concept of planet that is broad and inclusive of objects in this and other solar systems.

It makes no sense to have one definition of planet for this solar system and a different definition (or no definition) for other solar systems... The point is, a bad definition is worse than no definition.

I think you missed my point of an operational definition. If I define a planet as something in the solar system that has reached hydrostatic equilibrium, for example, then other scientists can look at my research and repeat the experiments or whatever it is scientists do. If my operational definition, however, is something Jupiter like that orbits a star, likewise that allows other other scientists to compare research on exo-planets and gas giants in the solar system.

In both cases the definition is not good or bad, it just allows scientists to compare apples with apples. So it makes a lot of sense to apply whatever definition works in any case, so long as you're clear what you mean. It makes no sense to call an operational definition a bad definition, and it obviously is better than no definition because it allows others to check the research and repeat it.

Whoops, in my previous comment I said something like "then other scientists can" which might imply I'm a research scientist. I'm not. I'm just a lowly code monkey with a science degree and an interest in science. :)

"I'm pursuing a degree in astronomy and have as much right to discuss this online as anyone else"

That depends on what you mean by "online." On your own blog or website, of course you do; on other people's, of course you don't - no one does. Nobody has a "right" to talk about any old thing at any old length on Russell's site (or on mine, or on anyone else's).

You've misunderstood (or simply ignored) the point of Russell's post, and just seized the word "Pluto" to justify yet another long boring self-obsessed rant about the status of Pluto, which, if I'm not mistaken, no one here gives a shit about except in the way Russell discussed it - as a matter of changing and expanding knowledge.

"Just because something is called a "planet" doesn't mean it is a planet."This exactly what the IAU is saying - a dwarf "planet" is not a planet. Just like a Koala "Bear" is not a bear. The IAU in their resolution appears to use quotes around 'dwarf planet' to separate it from real planet.Now you may disagree with the definition of planet and it does seem a bit arbitrary. If they called them something else (Small Hydrostatic Body?) would that have been fine? Do these 'dwarf planets' need to be some sort of planet? Why not call everything orbiting the Sun a Solar System Body and then attach different modifiers like Giant and Dwarf?It is not like any of this is set in stone and won't change over time. Look at the enormous change in organismal classification since Linnaeus - definitions and classification schemes are always being modified.

"Look at the enormous change in organismal classification since Linnaeus - definitions and classification schemes are always being modified."

Just what I was thinking - like the arguments about what exactly a panda is. And that stuff is interesting - but impassioned frenzies over it are not. The subject is knowledge, not politics. The epistemology is interesting; the politics is utterly boring. That's what M&K didn't get.

***Interestingly, I've met Tyson; he autographed a copy of his book for me, and we were able to laugh and joke about our differences. I don't know why others cannot do this.***

Perhaps it's because you're not visiting blogs in order to "laugh and joke about differences." You bombard them with Epic Drama. Also, it's not so easy to get away from a conversational partner - or to tell her what you're really thinking - when she's standing in front of you at a convention asking for a book autograph.

Yes, you have every right to express your opinion. Others have every right to find you tedious.

I can say is that I couldn't care less whether Pluto is a planet or not; it changes nothing about the nature of it or any other body in the universe. But every time a site I read mentions Pluto and Ms Kornfield shows up it makes me want to convince the IAU to never make Pluto a planet again. It is very difficult to define natural objects in a meaningful way - too much variation.

Feel free to contact Tyson, but please, don't project your sentiments onto him or other people. He happens to genuinely enjoy discussing this topic with people of all viewpoints.

A better classification scheme than one that says dwarf planets aren't planets are needed. That view is shared by many scientists. Attempts to stifle debate just because some hoped the 2006 IAU decision was the last word will not stop ongoing efforts to get a better definition that will genuinely expand knowledge of planetary science and the solar system.

Laurel, I noticed your long disquisition that you edited and removed, before you removed it. You droned on about Ophelia attacking you and bossing you around, you told me I had foisted "an outright lie" on you when I was merely speculating (sarcastically and snarkily, yes), etc. Then you ended with "I won't go away, ever."

If you thought better of that post, then good. It's exactly the sort of thing I was referring to as Epic Drama from you. This is what turns people off to any point of view you might have. If you want people to take your points seriously, it's a good idea to try not to come off as disproportionate and completely ruled by your emotions.

Laurel has insightful and well-thought out comments in this blog. Those who respond by criticizing her stance just miss the whole point here. In my opinion, the idea of what "is" a planet should be based on scientific fact AND cultural useage. For whatever reasons- political, scientific, or personal, some in the IAU decided they didn't want Pluto to be a planet anymore. So they tried to get the down-grading accomplished in approximately 1996. The did not succeed in the demotion because the scientific facts didn't warrant it. After more planning and strategy, they decided "Hey, let's take a vote" and so it was set-up to happen on the last day of the 2006 symposium with a "show of hands."

As a lay person, I hope that the IAU or any other organization doesn't make a habit of making scientific decisions by a show of hands. Changing scientific ideals, words, or anything should not be done by a show of hands. I always relied on the facts to determine the truth. If they cannot agree on what a definition of a planet should be based on the facts, then it should not be changed until a sensible, scientific, and logical definition can be produced. The fact that they changed Pluto from a planet to a dwarf-planet, and subsequently a plutoid, shows this was ill-planned and unscientific.

Changing the definition of a planet is not the issue here. The issue is how they went about it and the fact that the definition these "so-called" brilliant scientists created is "flawed." (really wanted to use another word, but decided to keep it civil.)

I'd like to know how many people think the process the IAU used to define a planet was scientific. Even if you agree with their outcome, please be honest about the process!

The Plutophiles really need to get out more. In biology, we still don't have a universal definition of a species after even after hundreds of years of trying. This is for two reasons - variation and contingency -and gives us biological, phylogenetic, ecological and typological species to name a few. Then you take the higher categories - what is a genus or family? Ms. Kornfeld quest for an "environment-free" definition of planet is unachievable for the same reasons. As we gain more knowledge of the solar system and galaxy our definitions will change. Here are the largest known bodies in our solar system orbiting the sun (largest to smallest): Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Ganymede, Titan, Mercury, Callisto, Io, Moon, Europa, Triton, Pluto, Eris, Titaia, Makemake, Rhea, Oberon, Haumea, Iapetus, Umbriel, Ariel, Dione, Tethys, Ceres. Notice how several moons are larger than some planets. This means that Ganymede's classification as a moon is environmentally-dependent - if it were directly orbiting the sun instead of orbiting Jupiter it would be a planet. I read Triton may well be a denizen of the Kuiper belt captured by Neptune - does this make it a moon or a planet - was it a planet before it was captured? If Pluto and Charon were captured by one of the larger bodies what would they be?The IAU definitions may be bad, but just calling Pluto a planet solves nothing.

I must admit I didn't read Siobhan's comment too closely before my last post. I never knew that a conspiracy among 200+ astronomers robbed poor Pluto of its planethood. I must ask why they would feel the need to do such a thing - how would it benefit them? Did Clyde Tombaugh piss somebody off?I would also like to know what definition should replace the IAU's?

A better definition, supported by Dr. Alan Stern and other leading opponents of the IAU definition, is that a planet is any non-self-luminous spheroidal body in orbit around a star. Moons large enough to be in hydrostatic equilibrium can be considered "secondary planets," which they have been in the past. That encompasses both what they are and where they are.

With so many exciting exoplanet discoveries, even the definition above should not be considered the last word. Astronomers have discovered rogue planets orbiting no star at all, pulsar planets, planets orbiting binary pulsar-white dwarf systems, etc. The delineation between brown dwarfs and planets is also a subject of debate. When new data is coming in with such rapidity, it makes sense not to count any definition as "final."

is that a planet is any non-self-luminous spheroidal body in orbit around a star.

Luminous? I guess that means it gives off light. Light for physicists encompasses x-rays, gamma rays, radio waves, the visible spectrum, etc. Doesn't the definition you gave rule out Jupiter then? Jupiter gives off radio waves due to its powerful magnetic belts or something similar I believe.

Perhaps you meant any spheroidal body that orbits a star that doesn't have fusion happening at it's core? So, a contracting ball of hydrogen, with sufficient mass, would be a planet until it became a binary star? Seems arbitrary as any other definition.

"Perhaps you meant any spheroidal body that orbits a star that doesn't have fusion happening at it's core? So, a contracting ball of hydrogen, with sufficient mass, would be a planet until it became a binary star? Seems arbitrary as any other definition."

And if that were the definition, neutron stars and brown dwarfs would not be stars -- no fusion occurring. Laurel, do you see how making these definition is, to some extent, arbitrary? What's important is what's useful. Now consider that Pluto itself is not in orbit around the sun, but rather the Pluto/Charon system is. Should we call it a binary planet? Binary dwarf planet? What about other Kuiper belt objects we haven't discovered, ones possibly larger than Pluto? Consider Pluto/Charon's solar orbit -- you mention the fact that it's sometimes closer to the sun than Neptune, but I'd say that's a mark against it. The other planets have well-defined non-crossing orbits.

I can think of many perspectives from which it's useful to think of Pluto as different-in-kind from the planets -- in fact, I can't think of any scientific perspective from which it's useful to do so. Because it's round? That definition is so arbitrary as to be useless. It would be like defining the gas/liquid phase boundary as a particular temperature/pressure (which you would have to do individually for any substance with such a phase boundary) rather than as a description of the arrangement of the molecules (which is consistent from substance to substance with a few notable exceptions).

Anyway, why is it such a big deal to you? What exactly do you lose if Pluto is reclassified? Every person creates their own mental categorizations for phenomena -- if yours has Pluto as a planet, what does it matter whether people agree with you? There's no guarantee that your definitions will match everyone else's in any other domain of human life, so why do you think it has to be so in astronomy?

"I can think of many perspectives from which it's useful to think of Pluto as different-in-kind from the planets -- in fact, I can't think of any scientific perspective from which it's useful to do so."

Should read " -- in fact, I can't think of any scientific perspective from which it's useful to think of Pluto as a planet." Meaning that I fail to see how defining "planet" such that Pluto happens to be one gives us any meaningful advantage in studying, say, planetary formation. It seems to me that Pluto and Charon would have formed much like other Kuiper belt objects, while terrestrial planets would form like terrestrial planets, gas giants would form like other gas giants, and ice giants would form like other ice giants. Wouldn't it make more sense to classify Pluto/Charon as Kuiper Belt objects, at least until a bit of data suggesting that they're different-in-kind from those? Just one example of why I agree with Pluto's demotion.

Yes, by self-luminous, I meant an object that generates hydrogen fusion at its core, something Jupiter does not do.

I'm not sure what you mean by referencing "a contracting ball of hydrogen." Are you referring to a nebula contracting during the process of star formation? Gravitational collapse forms what is known as a "protostar," which has not yet begun hydrogen fusion. Are you asking if this should be referred to as a planet?

Neutron stars, pulsars, and white dwarfs are stellar remnants, essentially dead stars. They are classified as a type of star because they once conducted hydrogen fusion, something a planet never did.

Brown dwarfs are a fascinating area of the debate regarding the upper limit between a planet and a star. If I understand correctly, they are classed as "substellar objects" but still at the bottom of the "star" category because they fuse deuterium. They do illustrate that celesial objects don't fall neatly into categories the way we often want them to.

Pluto-Charon should be considered a binary planet system since the two objects orbit a common barycenter. If other spherical objects in the Kuiper Belt are discovered, they too should be classed as both planets and Kuiper Belt Objects, whether or not they are larger than Pluto.

The fact that Pluto crosses the orbit of Neptune for a brief portion of its orbit should not disqualify it from being classed as a planet. We have discovered at least one exoplanet system with two gas giants orbiting the same star in the same 3:2 resonance as Neptune and Pluto. If these are not planets, what are they?

From a geophysical standpoint, it is useful to view Pluto as a planet. The distinction of spherical versus non-spherical objects is not arbitrary; it is a significant factor in how the objects formed and the geophysical processes they are now experiencing. Spherical objects shaped by their own gravity have weather and geological processes such as plate tectonics and volcanism, and are geologically differentiated into core, mantle, and crust, just like the terrestrial planets and unlike much smaller asteorids and comets.

Because there is so much we are still learning about all these bodies, and because often objects in different categories can have overlapping features in common, the best way to "open up the solar system" is to avoid being too rigid in any definitions. That is the flaw of the IAU definition, which forever limits the number of planets in our solar system to eight in spite of similarities other bodies have with these eight bodies.

For a very enlightening and comprehensive discussion of planet definition, visit http://gpd.jhuapl.edu/ and listen to the audio transcripts of the Great Planet Debate, held at the Johns Hopkins University Applied Physics Lab in August 2008.

"Pluto-Charon should be considered a binary planet system since the two objects orbit a common barycenter. If other spherical objects in the Kuiper Belt are discovered, they too should be classed as both planets and Kuiper Belt Objects, whether or not they are larger than Pluto."

You say it as if it should be obvious, but I don't think you've actually made a very good case for it. It's hard to say, because your posts come across as lectures by an self-appointed expert rather than as one side of a discussion. (Curious, given that that's pretty much your criticism of the process by which Pluto was demoted -- experts not willing to engage with amateur science enthusiasts, etc.) Can you present your objections to the definition in a format that makes discussion a little easier?

"The fact that Pluto crosses the orbit of Neptune for a brief portion of its orbit should not disqualify it from being classed as a planet. We have discovered at least one exoplanet system with two gas giants orbiting the same star in the same 3:2 resonance as Neptune and Pluto. If these are not planets, what are they?"

I did not say the orbit alone disqualifies it. However, if it could be demonstrated that Pluto and Charon formed like other Kuiper belt objects and just happen to have obtained their orbit by historical accident rather than through whatever means caused such orbits in the exosolar system you mention, would you be willing to admit that the Pluto demoters have a point? In other words, is there any set of discoveries that could persuade you that you're barking up the wrong tree with your definition?

"From a geophysical standpoint, it is useful to view Pluto as a planet. The distinction of spherical versus non-spherical objects is not arbitrary; it is a significant factor in how the objects formed and the geophysical processes they are now experiencing."

It seems to me you're overstating your case. There are theoretical reasons to think that Pluto is differentiated, but very little is actually known about its structure or formation.

Also, I didn't say that the distinction between spherical and non-spherical is arbitrary; that's a clear definition. However, your insistence on using it as the exclusive, or at least most important, qualifier for planet-hood is arbitrary. You failed to address the analogy to phase transitions, which I think is an important point. Are there conditions other than planetary formation within the context of a solar system that can lead to mass accretion such that a body becomes spherical? If so, doesn't this definition require us to call this thing, very much unlike planets in many respects (we assume for the case of the thought experiment), a planet? Again, is there any sort of discovery that would convince you that your definition isn't workable?

"Spherical objects shaped by their own gravity have weather"

Mercury? Ceres? What's the weather like on those bodies? Pluto may have weather, but it doesn't seem coriolis driven like the weather on any of the larger planets. Of course, there's not enough direct observational evidence to say either way.

"and geological processes such as plate tectonics"

On Pluto? Citation?

"and volcanism,"

On Pluto? Citation?

"and are geologically differentiated into core, mantle, and crust, just like the terrestrial planets"

There's a case to be made for it, but it's still unknown whether Pluto has such a structure.

"and unlike much smaller asteorids and comets."

Supposing that we found there's a gray area where bodies that do not have the right balance of mass and fine structure such that they become spherical, but nonetheless have differentiated interiors similar to the planets. Would you be willing to rethink your definition?

From what I can tell, you would be perfectly happy to class Pluto as a dwarf planet providing dwarf planets are defined as a type of planet. This seems trivial to me. Why do you think it's so important?

Of course my position represents one side in an ongoing discussion. I would never claim it is anything else. The point is that the IAU definition falls into the same category--one interpretation versus gospel truth.

"Can you present your objections to the definition in a format that makes discussion a little easier?"

Could you elaborate on what sort of format that might be? Are you looking for a simpler or more complicated explanation? Again, I think listening to the Great Planet Debate would probably be more helpful in this area, as it would give you the opportunity to hear multiple perspectives from many different people.

Should classification of an object be based on how it formed and how it came into its current orbit? We may not know enough at this point to answer this question. The exoplanet recently discovered orbiting its star backwards is one example. Since it likely didn't form with the star in the usual process, but likely was captured, is it a planet? Maybe we need to hold off on being too narrow in answering these questions until we have more data.

Cryovolcanism on Charon, a recent discovery, is discussed here: http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1901.pdf

Pluto is believed to be geologically differentiated and to have geological processes like those of the terrestrial planets but admittedly, we will not know for sure until we get the data from New Horizons, only six years from now.

I would certainly be willing to rethink any definition based on new data. I also think we may need to add new subclasses to many categories of objects, including stars, planets, and planetoids, as we make new discoveries.

Yes, I would be perfectly happy to categorize Pluto as a dwarf planet provided dwarf planets are defined as a type of planet. Not only would this be consistent with the use of the term dwarf in astronomy; it would also open up the possibility of a greater variety of objects being considered planets, and it would be in accord with the intention of Dr. Alan Stern, who coined the term dwarf planet in the first place.

Two central points are what I see as important here. One is that an object be defined not just by where it is but also by what it is.

The second and more important point, is that science not be done by fiat; in other words, that the people don't just accept a change because an individual or small group decreed it so. People deserve to know that the IAU view is just one interpretation and that other views are equally legitimate.

I think we're actually closer to agreeing than it seemed at first -- you seem to have a way of convincing everyone that you're being combative. As far as I can tell, that's the source of what you deem "personal attacks."

"Yes, I would be perfectly happy to categorize Pluto as a dwarf planet provided dwarf planets are defined as a type of planet. Not only would this be consistent with the use of the term dwarf in astronomy; it would also open up the possibility of a greater variety of objects being considered planets, and it would be in accord with the intention of Dr. Alan Stern, who coined the term dwarf planet in the first place."

This fails to answer my question: why is it so important that dwarf planets are planets? Again, the distinction seems trivial to me. The reasons given are a) consistency with other uses of the word in astronomy, b) more objects would be planets and c) Dr. Alan Stern would really like it to be the case. On (a), I agree consistency is good, but then we get back to koala bears, panda bears, and many other examples of inconsistency in scientific notation. Basically, it's nice to have, but not essential to the question at hand. For (b), I think it's undesirable to have a lot of new bodies become planets. To me, the important thing about planets is that they're the gravitationally dominant objects in the solar system excepting the sun itself, and grouping them with a bunch of gravitationally insignificant (or nearly so) bodies doesn't make sense to me. For (c), well, I don't know Dr. Stern from Adam and I don't really care whether he gets his way or not.

"Two central points are what I see as important here. One is that an object be defined not just by where it is but also by what it is."

Epistemologically, I don't think that is even possible. The definition determines the object (in the sense of perceptual or conceptual figure, not the material substrate upon which that figure is built), not vice versa. But getting into epistemology in this kind of discussion always turns into a rat's nest, so it may not be productive to argue that point further.

"The second and more important point, is that science not be done by fiat; in other words, that the people don't just accept a change because an individual or small group decreed it so. People deserve to know that the IAU view is just one interpretation and that other views are equally legitimate."

It depends wholly on what you mean by "legitimate," and I think the "equally" is suspect even so. From this post, I'm getting the sense that the true debate is between people (like me) who think the term "planet" should be reserved for the gravitationally dominant bodies in a solar system versus people (like you) who think the definition of "planet" should be open to gravitationally less significant objects (as long as they're round). I suspect one or the other view is more useful than the other (two guesses which one), meaning that they aren't equally legitimate.

Still, I agree with you that more data is needed before we can see which definition will probably be more useful.

One more point -- I think it's a little misleading to present Cryovolcanism as a particular type of conventional volcanism. The causal nature of the two phenomena simply don't seem analogous in the way your presentation suggests.

Plus, as far as I can tell, comets experience something very much like Charon's Cryovolcanism without the benefit of being planets in the first place -- making it a pretty poor indicator of planet-hood.

"I think we're actually closer to agreeing than it seemed at first --you seem to have a way of convincing everyone that you're being combative."

I agree, and I apologize for sounding combative. I think I've spent too much time working for political campaigns. :)

"From this post, I'm getting the sense that the true debate is between people (like me) who think the term "planet" should be reserved for the gravitationally dominant bodies in a solar system versus people (like you) who think the definition of "planet" should be open to gravitationally less significant objects (as long as they're round)."

Yes, this is the crux of the debate, and it is where we have to agree to disagree. As is obvious, I believe that objects in hydrostatic equilibrium have more in common with planets than with asteroids and non-spherical KBOs. We may even end up with two definitions existing side by side. My main point is to refute the claim by some that the debate is over and that the IAU view is the only legitimate view.

Regarding Charon and cryo-volcanism, I will have to look into this further. I know cryovolcanism happens on some of the moons of the gas giants and that there is speculation that both Charon and Pluto may harbor subsurface oceans, potential habitats for microbial life.

The spherical moons of the planets have at times been referred to as "secondary planets" since geophysically, their composition is very much like that of the planets. I would like to see this category reinstated; we can continue to call them moons and satellites while technically defining them as secondary planets (as opposed to primary planets, which orbit the Sun directly).